This invention relates to apparatus for contacting surfaces with flowing gas, e.g. drying apparatus, particularly drying apparatus that provides means to supply drying gas to a surface to be dried and, then, provides means to remove the spent drying gas together with the moisture or solvents it has picked up, from the drying zone without allowing any substantial part of the spent gas to escape into the environment's proximate the site of the drying.
There are a large number of industrial drying processes wherein surfaces are coated with liquid-based compositions and wherein the liquid component of the composition must be removed immediately so that the process may be carried out at a commercially-suitable rate. Such processes require the use of large quantities of drying air continuously supplied to a drying zone.
Among such processes are those for drying printing inks to surfaces that are, typically, moving at hundreds of feet per minute. Other such processes are those wherein adhesives or other such functional coatings are being applied. The economics of such processes become particularly sensitive to the efficiency of the drying process when large quantities of solvents such as water must be removed and, especially, when expensive or noxious solvents must be removed upon drying. In such cases, it is necessary to provide a drying process wherein the spent drying gas (that is, the drying gas which has passed through the drying zone and picked up the volatile material being evaporated) is removed from the drying zone without escaping into the environment of the drying equipment. A substantial portion of the cost of such removal is the fuel for energy expended on driving exhaust fans.
Drying apparatus has been provided in the prior art which was intended to meet the general requirements of the process described above. In general, high-velocity drying air was sent, through a number of air tubes, into the drying zone and sufficient air outlet exhaust capacity was provided to remove the drying air, once spent, from the drying zone. In one example of the prior art, a jet of drying air was directed downwardly into the drying surface and thence deflected to form a vortex on each side of the jet before being exhausted. There are a number of problems with this earlier-existing equipment which may be summarized by reference to the fact that it was very expensive to provide the exhaust capacity to remove spent drying gas and, usually, this factor played an important role in limiting the overall operational efficiency of the drying process. For example, the speed of the drying process itself would often be limited to allow the existing exhaust system to achieve its purpose.
Attempts have been made to increase velocity, and volume of drying gas at the surface to be dried; the associated increases in equipment cost, cost of operating the equipment, and the increased difficulty of collecting spent gases, have made such slower operating rates generally acceptable practice.
Another drawback to prior art drying apparatus, and like gas-handling apparatus, is the disproportionate amount of space adjacent the surface to be dried which is taken up by structure associated with merely handling incoming and/or outgoing gas relative to the amount of space which is available for effective drying.
The present inventor directed his efforts to improving such gas-handling processes.